DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS CSG 15Research and Development

Final Project Report(Not to be used for LINK projects)

Two hard copies of this form should be returned to:Research Policy and International Division, Final Reports UnitDEFRA, Area 301Cromwell House, Dean Stanley Street, London, SW1P 3JH.

An electronic version should be e-mailed to resreports@defra.gsi.gov.uk

Project title An investigation of the causes and consequences of variable nozzle performance     

DEFRA project code PA1731

Contractor organisation and location

Silsoe Research InstituteWrest Park, Silsoe, Bedford, MK45 4HS          

Total DEFRA project costs £ 79,946

Project start date 01/10/01 Project end date 01/10/03

Executive summary (maximum 2 sides A4)

Work on spray measurement over a number of years has shown that some nozzles are variable in their measured performance. This may result in variability in flow rate, atomisation characteristics, deposition and drift. The main objectives of the work reported here were to evaluate spray measurement techniques and determine variation in spray characteristics for common nozzle types and spray liquids and to assess the consequences of variation in nozzle performance for measurements of spray drift in the wind tunnel and spray deposition. The research evaluated a range of spray measurement techniques and determined the variation in spray characteristics and behaviour over time. Common nozzle types and standard spray liquids were considered, and the consequences for measurements of spray drift in the wind tunnel and spray deposits on target surfaces evaluated.

Drop spectra were measured using a range of instruments that covered all the techniques currently in common use to characterise agricultural nozzles. Techniques included laser diffraction (Malvern Spraytec), imaging (Oxford Lasers VisiSizer) and phase Doppler analysis (Dantec PDA). A series of standard nozzles that included BCPC reference nozzles and reduced drift air-induction nozzles were used throughout the experiments. Spray liquids included water, solutions of a non-ionic surfactants, and emulsions. Measurements of drop spectra were made in the Silsoe Research Institute nozzle test chamber equipped with a computer controlled x-y transporter. Measurements of spray deposition were made in the Silsoe Research Institute re-circulating wind tunnel using its climate control facilities.

Statistical analysis of drop spectra data showed that with imaging and phase Doppler analysis techniques, where particles are counted and sized, a sub-sampling procedure know as boot-strapping could be used to determine the numbers of drops per sample that are required to provide a specified level of accuracy for a given statistic. Although the technique was found to generally be reliable, it is recommended that this procedure is not be used CSG 15 (9/01) 1

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

for measurements taken over long periods since there was evidence that long-term variability may occur and this may lead to an under-estimation of variability.

Although phase Doppler, imaging, and laser diffraction techniques all provided accurate measurements of sprays formed from true solutions, sprays formed from emulsions and air-included sprays were not accurately sized by the phase Doppler technique. This is because the technique relies on refraction and the optical signal is degraded by the presence of internal structures. It is suggested that the existence of air-inclusions within drops can be determined by examining the validation rate of the phase Doppler technique. For all the liquids tested, the imaging and laser diffraction techniques were in close agreement. It was concluded that these techniques are suitable for measuring air-included sprays such as are generated by air-induction nozzles. They have the capability of covering most of the sprays used in agriculture.

Solutions of the non-ionic surfactant Agral are currently the de-facto standard used for drop spectra and drift measurement. Drop spectra measurements using aqueous solutions of Agral showed a clear temperature effect associated with the cloud point of the solution. It appears that there is a step change increase in drop size as liquid temperatures rise above the cloud point. The increase in terms of VMD can be as high as 80 µm. For Agral at 0.1% concentration, perhaps the most commonly used solution in agricultural spray research; the cloud point temperature is around 31 °C. It was concluded that liquid temperatures could reach this value during laboratory tests and as a result unreliable results could occur. The effect of temperature was implicated in the lack of agreement between laboratories using similar equipment and instruments. Since Agral is likely to be unavailable in the near future, it is recommended that cloud point should be considered when a replacement non-ionic surfactant is discussed.

With the range of instruments and liquids used, humidity did not appear to significantly affect drop size generation or the measurement techniques. However, humidity clearly influences the rate of evaporation as drops disperse. Humidity changes were shown to influence the downwind deposition of drops in LERAP tests. However, LERAP classifications are comparative and the use of a reference spray operating within the same humidity range removed this influence. In effect, a reduced drift nozzle classified into a * rating by the LERAP procedure at a particular humidity will remain in that category irrespective of changes in humidity because its fractional reduction in drift remains the same. This demonstrates the robustness of the experimental protocol used for the LERAP tests.

From long time series data using laser diffraction, it appears that liquid temperature changes can be responsible for the long term variability of drop spectra measurements. It is therefore recommended that during drop spectra measurements liquid temperature is monitored and possibly controlled.

It is recommended that further work is carried out to investigate the relationship between dynamic surface tension effects, cloud point and atomisation behaviour to provide a more complete understanding of the effects of temperature on spray behaviour and measurement. Further work is also required on the effects of humidity on spray dispersion and drift to provide and more complete understanding of the role of climatic conditions in long range spray drift.

CSG 15 (9/01) 2

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Scientific report (maximum 20 sides A4)1 Introduction

Work on spray measurement over a number of years has shown that some nozzles are variable in their performance. This may result in apparently random changes in flow rate, atomisation characteristics, deposition and drift. There are several possible causes – those relating to fluid flow through the components of the spraying system and the break up mechanisms, which are likely to be chaotic processes, and those relating to environmental conditions such as temperature.

The main objectives of the work reported here were to evaluate spray measurement techniques and determine variation in spray characteristics for common nozzle types and spray liquids and to assess the consequences of variation in nozzle performance for measurements of spray drift in the wind tunnel and spray deposition. A second aspect to variability is the potential lack of repeatability between replicate measurements. For example, two consecutive measurements of spray characteristics are likely to yield similar values whereas if the spray nozzle is removed and replaced, or a new batch of liquid is required, or the second measurement is made after several hours rather than a few minutes, a significant change in the measured value often results. There are many potential causes of such variation, but the consequence is that only large differences in spray characteristics can be determined with any statistical significance. It is important to establish whether the measured change in drop size distributions between replicates results in comparable changes in performance characteristics related to drop size, such as spray drift.

The research evaluated a range of spray measurement techniques and determined the variation in spray characteristics and behaviour over time. Common nozzle types and standard spray liquids were considered, and the consequences for measurements of spray drift in the wind tunnel and spray deposits on target surfaces evaluated. The project set out to:

Evaluate spray measurement techniques. Investigate the variability in spray characteristics due to changes in temperature and humidity. Determine the variability in spray characteristics over time. Assess the consequences for measurements of spray drift in the wind tunnel and spray deposit.

2 Method and materials

2.1 NozzlesA series of sprays produced by five nozzles that cover the range used in common agricultural practice were used for most of the work reported here. Details are given in Table 1. Three of the sprays selected represented boundary nozzles for the original BCPC classification scheme (Doble et al 1985). The fourth and fifth nozzles (Hardi Injet 03 and Billericay BubbleJet 03) are an air-induction nozzles that have LERAP *** classification and can be regarded as low-drift nozzles. Of the 03 sized air-induction nozzles currently available in the UK, the Injet is the coarsest, and the BubbleJet the finest. It should be noted that at present the BCPC scheme does not include air-induction nozzles.

Table 1. Sprays used in experiments

Nozzle Pressure Descriptionbar

01F110 4.5 Boundary between Very Fine and Fine, BCPC classification03F110 3.0 Boundary between Fine and Medium, BCPC classification08F80 2.5 Boundary between Coarse and Very Coarse, BCPC classification

Hardi InJet 03 3.0 Air-induction nozzle producing very coarse drops (coarser than 08F80)Billericay BubbleJet 03 3.0 Air-induction nozzle producing very coarse drops (coarser than 08F80)

2.2 Drop measurement equipmentDrop sizing was carried out using three different laser-based techniques namely the Oxford Lasers VisiSizer which utilises an imaging technique (Murphy et al 2001), the Dantec PDA (Tuck et al 1997) which utilises phase Doppler anemometry; and the Malvern Spraytec which utilises laser diffraction (Kippax et al 2002). Imaging and phase Doppler techniques measure individual drops whilst laser diffraction measures the angular distribution of scattered light intensity and calculates an ensemble average of the particle size distribution responsible for that scattering. Phase Doppler anemometry takes a temporal or flux-based sample, whilst imaging and laser diffraction take spatial or concentration-based samples (Parkin 1993). Phase Doppler anemometry and measures drop velocity and particle size directly. Imaging can supply velocity information through the use of double pulsing. However, for the experiments reported here the VisiSizer was used in the sizing mode since the dynamic range in the size/velocity mode was insufficient to measure the full range of drop velocities encountered.

CSG 15 (9/01) 3

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

The instruments were operated in the SRI nozzle laboratory using an x-y transporter (Lake & Dix 1985) to traverse the spray cloud through the sample volume and ensure an adequate spatial sample. The nozzles were operated 350 mm above the sample volume for all instruments.

2.3 Environmental measurementsAll tests were performed in the test cell within the SRI nozzle laboratory. It is not air conditioned and the air temperature fluctuates, especially in the summer. Winter tests allow temperature to be varied through additional heating and experiments can also be carried out when ambient temperatures are high. Liquid temperatures were more easily controlled through heating or cooling of the liquid, although liquid delivery to the nozzles was from a pressurised liquid container through a delivery tube approximately 7m long. Liquid temperatures were measured in the container before pressurisation. When experiments were carried out with liquid temperatures other than ambient, they were carried out rapidly to avoid the liquid reaching to room temperature. A Hanna Instruments HI8564 thermo hygrometer was used to measure the air temperature and relative humidity. It was positioned 150 mm directly above the nozzle. In this position the sensor measured entrained air close to the nozzle. Liquid pressure was monitored at the nozzle by a calibrated digital transducer.

During wind tunnel experiments on deposition humidity was monitored by a Michell Dewmet cooled mirror dew point meter and temperature by a Tempo digital thermometer. Wind speed was monitored using Solent Research Model ultrasonic anemometer.

2.4 Wind tunnel experimentsSpray drift and deposition experiments were carried out in the Silsoe Research Institute re-circulating wind tunnel. A fixed air velocity of 2 m/s was used for all tests with a nozzle height of 0.6m. Spray drift was collected using an array of 2 mm diameter polyethylene lines positioned at 0.1m height and 2, 3,4,5,6 and 7 m downwind of the nozzle. The lines covered the full working width of the tunnel. The spray liquid was an aqueous solution of 0.2 % w/v Green-S dye with 0.1% v/v Agral non-ionic surfactant. Collectors were exposed to spray for 10 s using a solenoid valve and electronic timer. This gave a practical working range of dye concentrations when lines were washed with 10 ml of water. Dye concentration was measured using a spectrophotometer and the deposits normalised via a tank sample.

The SRI wind tunnel has systems for increasing and decreasing the humidity. The dehumidification facility consists of a by-pass circuit with a cooling coil, circulation fan and re-heat coil. The system has the capacity to remove a nominal 18 l/hr of water from the tunnel air flow making it possible to operate below ambient humidity but with performance dependent on atmospheric conditions. With airflow circulating in the tunnel at 2 m/s and maximum cooling, steady state conditions are reached in ~ 15 min. The humidification system consists of a series of twin-fluid nozzles that inject a fine mist into the main recirculation.

To process the data from the wind tunnel experiments the scaling procedure outlined by Walklate et al (1998) was used to calculate the length scale of spray drift from a 12 m (24 nozzles) boom sprayer. Assuming a buffer zone distance of 6 m the relative Predicted Environmental Concentrations (PECs) for the field scale treatments were determined and the LERAP star rating established. Reductions in drift from the standard in the range 50-75% receive a * rating; 25-50% a ** rating and <25% a *** rating (Gilbert 2000). By grouping the data according to nozzle and humidity the LERAP star ratings for each nozzle and humidity combination could be determined and compared.

3 Results3.1 Evaluation of spray measurement techniques

3.1.1 Statistical analysis of drop sizing The main emphasis of this section of the work was to ascertain the minimum number of drops required to be measured to classify an agricultural nozzle such that when operating conditions (e.g. pressure or liquid) changed, it would be possible to measure performance those changes with a degree of statistical certainty. Although some of this work was carried out using the Dantec phase Doppler analyser (PDA) most of this work was carried out using the more modern instrument, the Oxford Lasers VisSizer. Because the Malvern Spraytec uses ensemble averaging, rather than individual particle sizing, the numbers of drops sampled during a measurement is not relevant to that technique.

In mono-dispersed sprays the number of drops required for statistical validity may be fairly small, but agricultural sprays are generally poly-dispersed with a wide range of drop sizes for a given source and a wide range of sizes depending on the source.

Wagner & Drallmeier (2001) suggested a method of determining the number of drops required for accurate measurement using a “boot-strapping” technique where-by a single continuous sample is broken down into sub-samples of different duration (and hence numbers). By randomly selecting the start time from within that large sample, the variation for a specific number of drops can be obtained. By varying the sample duration, a variation of measurement performance with sample numbers can then be obtained. This technique maybe very good in some applications, but within agricultural, the position within the spray plume may have a bearing on the outcome since the size variation within the plume can be vary large, especially for nozzles that produce large drops. A similar methodology had been used at SRI for some time, but in this case, instead of a static continuous sample, the spray plume is scanned

CSG 15 (9/01) 4

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

backwards and forwards across a representative portion of the spray plume to obtain many discrete samples. Each sample or scan can then be analyzed individually and by combining increasing numbers of consecutive scans, a picture of the relationship between drop numbers and performance measurements can be obtained.

The scan length chosen for experiments with the VisiSizer was sufficient for the spray to clear the equipment sample volume at each edge and the scan speed was selected for the nozzles such that on average approximately 500 drops were obtained with each scan. To ensure scan speed had no effect on measurement performance, an investigation with the first four test nozzles where scan speed was varied in the range 2 to 50 mm/s (with the number of scans adjusted to ensure a minimum of 25,000 were obtained) indicated no influence from this parameter.

In subsequent tests with the Oxford Lasers VisiSizer a scan speed of 50mm/s was used; the maximum possible with the nozzle transporter. The total number of scans made for each nozzle depended on the data acquisition rate which varied from nozzle to nozzle and with each measurement device. However, the minimum criteria were 100 scans as long as at least 50,000 drops had been obtained. Analysis consisted of firstly analyzing each individual scan to obtain a performance measurement, Volume Median Diameter (VMD), and corresponding number of drops in the sample. It is then assumed that from the complete measurement, an individual measurement could have started at the beginning of any of the individual scans and continued for as many consecutive scans as desired. Hence, by re-analyzing the data over and over again and each time starting at a different individual scan and proceeding for an increasing number of scans each time, a relationship between number of drops and VMD can be built up.

Fig. 1 Relationship between VMD and number of drops in sample derived from the ‘boot-strapping’ process for the 08F80 nozzle at 2.5 bar – Oxford Lasers VisiSizer

Fig. 2 Effect of drop numbers on the max/min variation of VMD expressed as a percentage of the overall mean VMD for four test sprays – derived form a boot-strapping process using drop imaging (Oxford Lasers VisiSizer).

Fig. 1 shows a typical relationship between VMD and number of drops in the sample derived from this bootstrapping process for the 08F80 test nozzle. A measure of the variability between replicates for a parameter is the maximum value minus the minimum value

CSG 15 (9/01) 5

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

expressed as a percentage of the mean for that parameter. Fig. 2 shows the effect on VMD for four test sprays and their relationship with drop numbers for the boot-strapping process as measured with the Oxford Lasers VisiSizer. The results showed a steep initial decrease in variability followed by asymptotic behaviour. The point above which asymptotic behaviour began was between about 10,000 and 15,000 drops. Above the number many more drops had to be obtained for only small improvements in accuracy. Additionally, measuring more drops would require longer measurement times and this may introduce longer term variability due to nozzle and/or environmental fluctuations. The degree of variability was found not to be a function of drop size since the most variable spray, produced by the 08F80 nozzle, had a lower VMD (410 µm) than the InJet 03 (574 µm).

To examine the practical relevance of the boot-strapping method, a further test was carried out with the test nozzles whereby each nozzle was scanned in the same way as for the boot-strapping test, but at different scan speeds which had the effect of varying the drop numbers. These tests were replicated 10 times and the max/min variation in VMD as a percentage of mean VMD compared with those values for the boot-strapping process. Also, for all except the 01F110 nozzle a full BCPC scan was used at 3 different scan speeds to give an indication of variation for a different scan pattern, although these tests were only replicated 3 times. (It was expected that more replication would result in lower variability in mean values). The results for these tests are added to those represented in Fig. 2 and are shown in Fig. 3.

The results from these two other tests tend to indicate that the boot-strapping technique is reliable in the early stages but then tends to under-estimate the degree of variability for larger drop samples, thus confirming the earlier indication that too big a sample might not be desirable.

Fig. 3 Comparison of results from a boot-strapping process with conventional methods of obtaining data using drop imaging (Oxford Lasers VisiSizer).

3.1.2 Suitability of drop sizing instrumentation for measuring air-included sprays & emulsions.It is well known that optical drop measurement techniques relying on refraction for signals are not suitable for the measurement of sprays where the drops have internal structure (e.g. Schuch & Wreidt 2001). This particularly affects the phase Doppler technique employed by the Dantec when used in forward scatter. It has been suggested that this might also affect laser diffraction, but recent work by Kippax et al (2000) showed that this was not the case for agricultural nozzles since refraction only becomes an issue with transparent particles <5 µm diameter where the Mie approximation is employed. However, the extent of the problem relates to air-induction nozzles, where drops contain air inclusions (Tuck et al 1997), was not clear. To investigate the characteristics and symptoms exhibited by various techniques when inclusions are present, a series of comparative measurements were made using two standard nozzles in Table 1 (i.e. omitting the 01F110) and the using Dantec phase Doppler analyser. Four liquids were used; water, a

CSG 15 (9/01) 6

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

0.5% aqueous solution of Ethokem (surfactant) and an oil in water emulsion 0f 1.0% Actipron. The parameter selected to indicate the quality of the measurement was the overall validation rate (including the sphericity check). This data is shown in Fig. 4.

Fig. 4 Validation rates from the Dantec PDA (phase Doppler analyser) using conventional (03F100, 08F80) and air-included nozzles (BubbleJet 03, Injet 03) with water, a surfactant solution (Ethokem), and an oil in water emulsion (Actipron)

The flat-fan nozzles (03F110 and 08F80) with water had relatively uniform and high validation rates across the spray plume. Unexpectedly, the 08F80 had higher validation rates than the 03F110 nozzle; possibly because it is coarser and therefore less likely to exhibit coincidence errors caused by several drops traversing the measurement volume at the same time. The flat-fan nozzles with the water soluble Ethokem had slightly lower validation rates than with water, especially for the 03F110 nozzle where Ethokem produced a higher number of smaller drops. However, the same nozzles and the emulsion (Actipron) gave poor validation rates presumably because the emulsion particles in the drops disrupted the signal (Wreidt & Schuh 2002). This effect was more significant with the 08F80 nozzle since there are more coarse drops in this spray containing more emulsion particles and therefore more potential to disruption of the signal. The edges of the spray plume gave lower validation rates because there are fewer drops in this area.

The air-induction nozzles had lower validation rates with all liquids. Even with water the validation rate was less than for flat-fan nozzles. This suggests that even with pure liquids larger drops may contain a few transient air inclusions. With the water soluble Ethokem air-induction nozzles are known to produce drops with air-inclusions (Butler Ellis et al 1997) and this may disrupt the signal through multiple internal reflections. This will have a similar effect on disrupting the signal as for flat-fan nozzles. The coarser of the two nozzles (the Injet) also showed a reduced validation rate.

When comparing the three different measuring techniques - phase Doppler (Dantec PDA), imaging (Oxford Lasers VisiSizer) and diffraction (Malvern Spraytec) – it can be seen that the imaging and diffraction techniques are in close agreement. Results from the four nozzles and three liquids with the three measurement techniques are shown in Fig. 5. The only difference being that the BubbleJet which had lower VMDs than the 08F80 with the Malvern. This could be a temperature effect since the experiments were not carried out at the same time, or perhaps connected with instrument range. The Oxford Lasers VisiSizer was adjusted to a lower detection limit of 60µm diameter. It was therefore less accurate for the smaller drops, whereas the Malvern was adjusted to measure up to 1200 µm diameter but this instrument has large geometrically graded bins in the upper range reducing its accuracy for coarse sprays. The results from the phase Doppler system are comparable with the others but only for using flat-fan nozzles not the air-

CSG 15 (9/01) 7

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

induction nozzles. As expected the Dantec phase Doppler analyser gave completely different results for both the water soluble surfactant (air-inclusions) and the emulsion.

Fig. 5 Comparative drop spectra measurements using phase Doppler (Dantec PDA), laser diffraction (Malvern Spraytec) and imaging (Oxford lasers VisiSizer) techniques with sprays from conventional (03F100, 08F80) and air-included nozzles

(BubbleJet 03, Injet 03) with water, a surfactant solution (Ethokem), and an oil in water emulsion (Actipron).

3.2 Evaluation of variability due to changes in temperature and humidity.

3.2.1 Drop size measurements using laser diffractionTo investigate temperature effects the Malvern Spraytec was set to take measurements in the ‘flash mode’ using a 100 Hz data sampling speed with the data averaged and presented at 10 Hz intervals. Since this equipment has an effective cylindrical sample volume of 500 mm length (the vignetting range) by the beam area, a single nozzle pass along the complete spray fan width is sufficient to characterise the treatment. A scan speed of 20 mm/s was used. Since at the test height used, 350 mm, a full scan takes about 60 seconds for 110° fan angle nozzles, this is sufficient to obtain an accurate measurement of the flow rate during that treatment. This high sample rate enables an accurate time history of the nozzle performance to be examined, and this can subsequently be translated into a positional performance. Knowing the start and end positions at which the spray is measured also enables an estimate of the fan angle for the particular nozzle to be made. This experiment was carried out when the ambient (air) temperature varied between 25 oC and 31 oC and used all four test nozzles spraying two solutions; water only and a 0.1% solution of the non-ionic

CSG 15 (9/01) 8

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

surfactant Agral. Liquid temperatures as measured in the containers varied between 7 oC and 47 oC. The relative humidity ranged from 41 % to 77 %. This varied during tests since they were carried out in a confined space.

The results of these tests are shown in Fig. 6; the standard flat-fan nozzles and Fig. 7; the air-induction nozzles. When spraying the 0.1% Agral solution both of the flat-fan nozzles exhibit a step change in performance when the liquid temperature reached about 30 oC. It has been suggested that at a temperature of 31 oC Agral reaches its ‘cloud point’ when the normally clear solution becomes cloudy. This was confirmed when heating a small quantity of the solution which was observed to turn cloudy between 30 oC and 31 oC. This is a common effect exhibited by many non-ionic surfactants (see http://www.surfactant.co.kr/surfactants/cp.html). Reducing liquid temperatures below this ‘cloud point’ also appeared to increase VMD, especially with the 08F80 nozzle. Liquid temperature also appeared to have a considerable effect when spraying water only, especially with the coarse 08F80 nozzle. For both of the flat-fan nozzles above a critical temperature there appeared to be very little effect on VMD; this temperature was about 28 oC for the 03F110 and 22 oC for the 08F80. Below the critical value there was an increase in VMD with decreasing liquid temperature and in particular with the 08F80 the increase was 42 µm when the liquid temperature fell from 22 oC to 8 oC. Since this nozzle is a boundary nozzle for the BCPC grid classification system, this effect could have serious implications for the measurement of nozzle classification. The BCPC protocol does not specify liquid temperature properties. In the SRI laboratory where the usual procedure is to fill liquid containers direct from the tap, it was found that on a particular day, the liquid temperature could vary from 6 oC to about 24 oC depending on how much liquid was used in the preceding time period. The delivery pipe runs from outside the building below ground and then through the roof of the heated building. With the air-induction nozzles, Fig. 6, there are again significant effects on VMD with liquid temperature and again both nozzles exhibit steep changes at around the ‘cloud point’ with the Agral solution although the degree of change is less than for the flat-fan nozzles.

Fig. 6 VMD vs liquid temperature for flat-fan nozzles using laser diffraction (Malvern Spraytec)

CSG 15 (9/01) 9

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Fig. 7 VMD vs. liquid temperature for air-induction nozzles using laser diffraction (Malvern Spraytec)

3.2.2 Drop size measurements using imagingTo check the validity of the measurements made with the Spraytec, the Oxford Lasers VisiSizer imaging system, which is unlikely to be affected by thermal gradients, was used to verify the ‘cloud point’ affect using the 03F110 nozzle spraying the 0.1% Agral solution. Two replicates were completed at each of two liquid temperatures, 12 oC mean and 36 oC, the air temperature being 22 oC, Fig. 8.

Fig. 8 VMD profile across the width of a spray fan for 03F110 nozzle at 3bar spraying 0.1% Agral using imaging (Oxford Lasers VisiSizer) and laser diffraction (Malvern Spraytec)

CSG 15 (9/01) 10

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

The results shown in Fig. 8 showed good agreement between the two measuring systems not just in the increase in VMD due to the temperature, but also in the mean measured profile across the spray fan. This suggests that the temperature induced changes shown above are real and not connected to some as yet unknown characteristic of the system of measurement.

3.2.3 Further drop size measurements using laser diffractionThe first series of experiment using laser diffraction had been carried out whilst there was a very high ambient temperature (average about 27 °C). The possibility of obtaining similar results under different ambient temperatures was raised. For this reason a more limited experiment was carried out at a time when the ambient temperature could be held at a lower temperature level. In this experiment only the 08F80 nozzle was used spraying both water and the Agral solution since this nozzle gave the most variation in performance due to liquid temperature with both liquids. Ambient temperature although steady for each liquid treatment, rose between the two treatments from 16 oC to 18 oC. The results for the 0.1% Agral solution are shown in conjunction with the results from the earlier experiment in Fig. 6.

The results from the second series of experiments with the lower air temperature showed that there may have been a slightly higher ‘cloud point’, but this may be because the effect of the lower air temperatures on the liquid running through the long delivery pipe reducing the temperature at the nozzle. This would result in the liquid temperatures shown being higher than the nozzle temperature.

Fig. 9 Effect of liquid temperature on VMD for 08F80 nozzle at 2.5bar spraying 0.1% Agral measured using laser diffraction (Malvern Spraytec)

Fig. 10 Effect of liquid/air temperature difference on VMD for 08F80 nozzle at 2.5bar spraying water only – Spraytec

The effect with water in this experiment was similar to the earlier experiment, although the point at which the VMD started to rise appeared to be offset by the difference in ambient temperature. When the VMD was plotted against the difference between liquid

CSG 15 (9/01) 11

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

temperature and air temperature (Fig. 10), the curves appeared to be similar. This would indicate that when spraying water, the nozzle performance is dependant on the difference between liquid and air temperatures. This suggests that, for this nozzle, provided the liquid temperature minus the air temperature is greater than about -5 oC then performance will be reasonably stable. Below this value small temperature differences could result in large variations in VMD measurement.

3.2.4 Discussion on temperature effects on drop sizeThis liquid temperature effects see above may explain why three laboratories around the world which have recently measured the BCPC reference grid nozzles using similar equipment (Malvern laser diffraction analysers) have obtained very different results, especially for the coarser nozzles. The results are shown in Table 2.

The 08F80 nozzle used for the above work was of plastic (Kemetal) construction and to avoid to possibility of the material having an affect, a check test was carried out using the stainless steel BCPC 08F80 reference nozzle (the very coarse/extra coarse reference nozzle 10F65 was also included in this test). Table 3 indicates that the material type appears to have no influence on the outcome and also that other similar large orifice nozzles can be influenced by liquid temperature.

Table 2 VMD (µm) for BCPC reference nozzles at different laboratories using similar nozzles and laser diffraction instruments.

Reference Nozzle

BCPCBoundary

Womac [2000]

SRI CPASUniversity of Queensland

11001 VF / F 107 121 116 11003 F/ M 165 171 237 12006 M / C 247 228 307 08F80 C / VC 360 301 459

Table 3 Effect of liquid temperature for the two coarse BCPC stainless steel reference nozzles.

Reference Nozzle

Boundary Liquid temperature

°C

Airtemperature

°C

VMD

µm08F80 C/VC 6 24 348

18 24 311

10F65 VC/EC 6 24 405 15 23 376

3.2.5 Influence of humidity on drop sizeExperiments were carried out over a period of four months during which laboratory conditions changed quite markedly. Relative humidity was measured during these experiments as well as the air and liquid temperatures. This enabled the effect of humidity to be ascertained on performance of the test nozzles. To enable the effects of humidity on drop evaporation to be quantified the wet bulb depression, ΔT (difference between wet and dry bulb temperatures) was calculated. The results are shown in Fig. 11. This indicates that humidity had no discernable effect on drop spectra over the range of temperatures and humidity experienced.

CSG 15 (9/01) 12

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Fig. 11 Effect of humidity on nozzle performance measured using laser diffraction (Malvern Spraytec)

3.2.6 Implications of temperature effects on tests over long timescalesTo investigate variability over long timescales, repeated nozzles tests, where the liquid and air temperatures and relative humidity were recorded, were carried out over a one hundred day period. The temperatures measured during this period are shown in Fig. 12. Since the tests were carried out using all four nozzles and replicating six times first with one fluid and then with another, there are some variations in temperature profiles between the two fluids used (e.g. differences between morning and afternoon runs or one day to the next). The resulting VMD profiles are shown in Figs 13 and 14 with error bars indicating the standard deviation on the mean. For the two flat-fan nozzles the variations with time between the two fluids, Fig. 14, are very similar. Also for the 03F110 the variation between individual dates for a particular fluid is not significant. However, for the 08F80 nozzle the day to day variation can be significantly different (Students T-test). This could be caused by two problems; firstly the significance of air temperature on flow rate and VMD and secondly the cut-off point in the VMD/liquid-air temperature relationship at which VMDs below a figure of about -5oC rise quite steeply. Fig. 16 shows the VMD against liquid minus air temperature for the data obtained in different experiments. The liquid temperature points are individual replicates and the long term data is the mean for each of six replicates. There appears to be consistency between the results from the two nozzles spraying water. With 0.1% Agral solution the data below the ‘cloud point’ appears to show no cut-off point although there is an increase in VMD with increasing temperature gradient (Fig. 16).

CSG 15 (9/01) 13

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Fig 12 – The effect of fluid temperature variation on drop spectra measurements during long term test using laser diffraction (Malvern Spraytec)

Fig 13 - Variation of nozzle performance with time for two flat-fan nozzles using laser diffraction (Malvern Spraytec)

CSG 15 (9/01) 14

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Fig. 14 Variation of nozzle performance with time for two air-induction nozzles using laser diffraction (Malvern Spraytec)

For the two air-induction nozzles, Fig 14, there is little consistency between the water and 0.1% Agral treatments. This can be explained with reference to Fig. 7 where the Agral treatments exhibited a steady rate of change up to the ‘cloud point’. However, with the water treatments both exhibit a brief rise in VMD at a critical point around 22 oC liquid temperature. When the performance of the two air-induction nozzles is plotted as VMD against temperature gradient (Fig 16) it can be seen that the VMD from the BubbleJet increased to the point where the liquid and air temperatures were within +/- 5oC. The Injet results may in fact be similar, but the data is not consistent enough to prove this point. The results from the long term experiment with the 0.1% Agral solution and the two air-induction nozzles showed lower VMDs than found in the liquid temperature experiment. The reasons for this are not clear; it may possibly be due to the air temperature effecting flow rates or simply random variability.

Fig. 15– Performance of flat-fan nozzles with liquid minus air temperatures using laser diffraction (Malvern Spraytec)

CSG 15 (9/01) 15

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

Fig. 16 Performance of air-induction nozzles with difference between liquid & air temperatures using laser diffraction (Malvern Spraytec)

The variation in the individual measurements when using the two different liquids is shown in Table 4 using the mean standard deviation of the 6 replicates for each experiment carried out in the long term nozzle performance trial. This indicates that measurement variation is considerably greater with water for all sprays except the 03F110. Variation of performance with the 0.1% Agral solution is relatively constant up to the ‘cloud point’ for all the nozzles measured. However, with water the flat-fan nozzles exhibited a sharp rise in VMD when the liquid temperature was more than 5 oC lower than the air temperature, but with the air-induction nozzles, the VMD increased when this difference is between -5 oC and +5 oC. Since for this series of experiments the liquid temperature was generally in the region of between 2 oC and 8 oC below the air temperature, the variation could be expected to be more than for the Agral.

Table 4. Mean standard deviation of VMD from 15 experiments using four test nozzles and laser diffraction (Malvern Spraytec) replicated 6 times.

03F110 08F80 BubbleJet 03 Injet 030.1% Agral 1.144 1.644 2.453 2.754water only 1.088 2.454 3.806 6.172

3.2.7 Effect of humidity on drift and deposition in buffer zones

Although humidity was shown not to directly influence initial drop spectra it was thought likely to influence local dispersal, drift and deposition into buffer zones. The first four sprays in Table 1 were used in the experiments. Different levels of humidity in the wind tunnel were obtained by using ambient conditions, using maximum dehumidification, and humidification to obtain near saturation. Experiments with each spray/humidity combination were replicated four times.

Table 5 shows that, for all the nozzles tested, the mean length scale of spray drift increased with increasing wet bulb depression (ΔT). However, it is not clear how this influences the LERAP star rating calculation. To reduce the risk of errors caused by variations in evaporation in LERAP wind tunnel tests, the standard operating procedure requires the relative humidity of the tunnel to be above 80% (i.e. ΔT <3°C). In Table 6, the LERAP star ratings for the air-induction and Coarse/Very Coarse nozzles were calculated using two bases; by comparing the results to the standard nozzle at the same humidity; and by comparing to the results of the standard nozzle at the high humidity. Within a given humidity range the LERAP star ratings remain the same suggesting that controlling humidity is not critical for LERAP wind tunnel tests as long as the reference standard results are taken within the same humidity

CSG 15 (9/01) 16

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

range. However, it appears from using ΔT <3°C as a reference, that changes in humidity can alter drift equivalent to one or two LERAP star ratings depending on nozzle. Furthermore the results suggest that the air-induction nozzles are less sensitive to changes in humidity than conventional nozzles. This was confirmed by regression analysis (Parkin et al 2003)

Table 5. Influence of humidity on the calculated length scale (m) ± SE of drift from a 12 m boom

Drift length-scales for

three humidity categories

Nozzle & pressureIncreased

ΔT <3°C

Ambient

ΔT 3-7 °C

Reduced

ΔT 7- 10°C

01F100 @ 4.5 bar 12.01±0.76 14.11±1.59 20.64±3.33

03F100 @ 3 bar 6.00±0.51 7.70±1.20 12.01±2.16

08F80 @ 2.5 bar 1.40±0.11 2.47±0.48 3.53±0.88

Injet 03 @ 3 bar 1.64±0.12 1.97±0.28 2.25±0.38

Table 6. Influence of humidity on LERAP star rating for reduced drift nozzles referenced to the standard spray in same humidity range and in parentheses to the standard spray at high humidity (ΔT <3°C)

LERAP Star ratings for

three humidity categories

Nozzle & pressure Increased

ΔT <3°C

Ambient

ΔT 3-7 °C

Reduced

ΔT 7- 10°C

08F80 @ 2.5 bar ***(***) *** (**) *** (*)

Injet 03 @ 3 bar *** (***) *** (***) *** (**)

4 ConclusionsStatistical analysis of drop sizing techniques showed that with techniques such as imaging and phase Doppler analysis, where particle counting is involved, a boot-strapping procedure can be used to determine the numbers of drops per sample that are required to provide a specified level of accuracy. Although boot-strapping may generally be reliable, it should not be used for measurements taking over long periods since there is evidence that long-term variability occurs and this may lead to an under-estimation of variability.

Although phase Doppler, imaging, and laser diffraction can all provide accurate measurements of true solutions, emulsions and air-included sprays are not accurately sized by phase Doppler techniques. The presence of air-inclusions within drops can be determined by validation rate of the phase Doppler technique. Imaging and laser diffraction provide are in close agreement and can be used to size sprays of emulsions and air-included sprays as are generated by air-induction nozzles.

Agral is the de-facto standard non-ionic surfactant for drop spectra and drift measurement. Drop spectra measurements using aqueous solutions of Agral showed a clear temperature effect associated with cloud point. It appears that there is a step change increase in drop size as liquid temperatures rise above the cloud point. The increase in VMD could be as high as 80 µm. For Agral at 0.1% concentration the cloud point temperature is around 31 °C. If liquid temperatures reach this value during laboratory tests then unreliable results are likely to occur. The effect of temperature is implicated in the lack of agreement between laboratories using

CSG 15 (9/01) 17

Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

similar equipment. This effect has implications for the choice of surfactants for test liquids. Since Agral is likely to be unavailable in the near future, cloud point should be considered when a replacement non-ionic surfactant is discussed.

Humidity does not appear to significantly affect drop size at generation or its measurement with a range of instruments. However, it clearly affects the rate of evaporation as drops disperse. This effect was shown to influence the downwind deposition of drops in LERAP tests. The use of a reference spray operating at the same humidity range removes this effect and ensures that the experimental protocol is robust.

From long time series data, it appears that liquid temperature changes could be responsibly for the long term variability of drop spectra measurements.

5 RecommendationsIt is clear that the different sizing instruments compared in this study are broadly comparable when measuring pure liquids and sprays without air inclusions. Care needs to be taken when selecting instruments for use with sprays where drops have internal structures such as inclusions and emulsion droplets. In particular there are considerable difficulties with phase Doppler techniques. Because temperature can significantly alter the measurements of sprays of non-ionic surfactant solutions, care needs to be taken when selecting test liquids and conditions. Because the “industry standard” product Agral is likely to be withdrawn in the near future, cloud point effects should be taken into account when considering its replacement.

It appears that validation rates may be used as a quality control indicator for phase Doppler measurements and this may also be an indicator for air-inclusions within a spray. Further work should be carried out to determine the relationship between phase Doppler validation rates, air-inclusion levels and drop size to determine whether or not this approach could be used to classify levels of air inclusion.

Measurements of the performance of nozzles should be carried out using liquids of similar temperature to avoid cloud point and other similar effects. Further work to investigate the relationship between dynamic surface tension effects, cloud point and atomisation behaviour are required to provide a more complete understanding of the effects of temperature on spray behaviour and measurement.

Further work on the effects of humidity on spray dispersion and drift are required to provide and more complete understanding of the role of climatic conditions in long range spray drift.

6 ReferencesDoble SJ, Matthews GA, Rutherford I, Southcombe ESE (1985) “A system for classifying hydraulic nozzles and other atomisers into categories of spray quality” Proceedings British Crop Protection Conference – Weeds, Bracknell, p1125-1134

Gilbert A J (2000). Local Environmental Risk Assessment for Pesticides (LERAP) in the UK. Aspects of Applied Biology 57, 83-90

Kippax, P; Parkin C S; Tuck C R (2002). Particle size characterisation of agricultural sprays using laser diffraction. Proceedings of ILASS –Europe 2002, Zaragoza, Spain. 9 –11 September

Lake JR, Dix A (1985) “Measurement of droplet size with a PMS optical array probe using an x-y nozzle transporter”, Crop Protection, 4, p464-472

Murphy SD, Nicholls T, Whybrew A, Tuck CR, Parkin CS, (2001) “Classification and imaging of agricultural sprays using a particle/droplet image analyser” Proceeding Brighton Crop Protection Conference – Weeds, 8D-3, p677-682

Butler Ellis MC, Tuck CR, Miller PCH (1997) “The effect of some adjuvants on sprays produced by agricultural flat-fan nozzles” Crop Protection 16 (1) p41-50

Parkin CS, (1993) "The measurement of spray droplets" Chapter 4 in "Application Technology for Crop Protection" Edited by E Hislop & G.A. Matthews, CAB International, Wallingford, England

Parkin CS, Walklate PJ, Nicholls JW (2003) “Effect of drop evaporation on spray drift and buffer zone risk assessments” Proceedings BCPC International Congress – Crop Science & Technology 3C-3, p261-266

Schuh R, Wreidt T (2001) “Computer programs for light scattering by particles with inclusions” Journal of Quantitative Spectroscopy & Radiative Transfer 70, p715-723

Tuck CR, Butler Ellis MC, Miller PCH (1997) “Techniques for measurement of droplet size and velocity distributions in agricultural sprays” Crop Protection 16 (7), p 619-628

Wagner RM, Drallmeier JA (2001) “An approach for determining confidence intervals for common spray statistics” Atomisation & Spray, 11 (3), p54-68

Walklate P J, Miller P C H, Richardson G M, Baker D E (1998). A similarity scaling principle for risk assessment of spray drift. 14th

Conference on liquid atomisation and spray systems, ILASS Europe 98, p 499-504

Womac AR (2000) “Quality control of standardized reference spray nozzles.” Transactions of the ASAE. 43 (1), p47-56.

Wreiedt T, Schuh R (2002) “The inclusion –concentration measurement of suspension droplets based on Monte Carlo ray tracing” Measurement Science & Technology 13, p276-279

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Projecttitle

An investigation of the causes and consequences of variable nozzle performance     

DEFRAproject code

PA1731

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